Implantable cardiac defibrillators (ICD's) are well known in the art. These devices, encapsulated in a conductive housing or enclosure, are generally implanted in a pectoral region of a patient and electrically connected to the heart with one or more electrode carrying leads. One lead includes at least one defibrillation electrode arranged to be positioned in the right ventricle. An arrhythmia detector detects ventricular arrhythmias, such as ventricular fibrillation. When such an arrhythmia is detected, a pulse generator delivers a defibrillating shock between the defibrillation electrode in the right ventricle and the conductive housing to terminate the fibrillation. Alternatively, such defibrillation devices may further include another defibrillation electrode arranged to be positioned in the right atrium or superior vena cava (SVC), hereinafter referred to as the SVC electrode, which may be electrically connected to the right ventricular defibrillation electrode. In this arrangement, the defibrillating shock is delivered between the commonly connected right ventricular and SVC electrodes and the conductive housing.
Ventricular fibrillation is an immediately life threatening cardiac arrhythmia. It requires immediate and effective defibrillation therapy. As a result, an ICD must be capable of providing a defibrillation shock having an output magnitude that is above the output level that is required to defibrillate the fibrillating heart chamber. This is known as the defibrillation threshold (DFT).
Being able to reduce the DFT is most desirable for a variety of reasons. First, all implanted devices, such as an ICD, are usually powered by a depletable power source, such as a battery. Reducing the DFT would thus serve to reduce the energy demands of the battery during defibrillation, thus extending the battery life. Also, the size of ICD's is largely dependent on the size of the battery. There is a constant desire to be able to make these devices smaller. If the energy requirements can be reduced as, for example, by reducing the DFT, the size of the battery and the ICD may ultimately be reduced. Most importantly, ICD's generally include a high voltage capacitor or capacitors that deliver the defibrillation shock. These capacitors must be first charged to the defibrillation output before the defibrillation shock can be administered. By lowering the DFT, the charge time of these capacitors may be reduced thus allowing the defibrillation therapy to be delivered more quickly. This is of course important because when a patient is experiencing a fibrillation episode, time is of the essence to preserve life. The present invention addresses this important issue of DFT reduction.